if you prefer cli, Kano told you how to change it. If you don't like cli, it is also possible to configure this via GUI.

Press Alt+F2 and enter "kdesu systemsettings", enter your root pw. Then you can start login manager settings (in Hellfire located in advanced tab). In login manager settings go to the last tab (in german it is "Vereinfachung" in english it must be something like simplification). There is a checkbox to enable/disable auto login.